And I found this problem from a Chinese forum: 【图片】部分士兵模型对应有点错误，谁可以反馈一下给官方荷兰的最高级戟兵和葡萄牙的最高级龙骑兵模型是用的第二高级的模型【帝国3吧】_百度贴吧. A huge thank to the user who found it.
Say for fortress age units, we had 2 tiers of models in the original game, with tier 1 for veteran and tier 2 for guard/imperial.
Now that we have three tiers: tier 1 for veteran, tier 2 for guard and tier 3 for imperial.
The problem is: the imperial version of a royal guard unit does not change the model. The results are as follows. Note all the units are in the imperial tier:
This is the shared imperial halberdier (tier 3 model)
This is the imperial royal guard halberdier for the Dutch (still tier 2 model, note the belt is gone)
This is the shared imperial dragoon (tier 3 model)
This is the imperial royal guard dragoon for the Portuguese (still tier 2 model, note the tassel)
An earlier post seem to refer to a very same problem, this time with skirmishers (different color scheme between tier 2 and 3 model, but imperial royal guard does not change the model)
I’ve done some easy modding and I believe this can be fixed by changing a few lines in the imperial upgrade technology.